Nonwoven fabrics have been known for many years. In one process for producing nonwoven fabrics, a fiber batt or web is treated with water streams to cause the fibers to entangle with each other and provide some strength in the batt. Many methods have been developed for treating fiber batts in this manner in an attempt to duplicate the physical properties and appearance of woven fabrics.
U.S. Pat. Nos. 5,098,764 and 5,244,711 disclose backing members for supporting a fibrous web during the manufacture of nonwoven fabrics. The support members disclosed in U.S. Pat. No. 5,098,764 have a predetermined topography as well as a predetermined pattern of openings within that topography. In one specific embodiment, the backing member is three-dimensional and includes a plurality of pyramids disposed in a pattern over one surface of the backing member. This specific backing member further includes a plurality of openings which are disposed in the spaces, referred to as "valleys", between the aforementioned pyramids. In this process, a starting web of fiber is positioned on the topographical support member. The support member with the fibrous web thereon is passed under jets of high pressure fluid, typically water. The jets of water cause the fiber to intertwine and interentangle with each other in a particular pattern, based on the topographical configuration of the support member.
The pattern of topographical features and apertures in the support member is critical to the structure of the resulting nonwoven fabric. In addition, the support member must have sufficient structural integrity and strength to support a fibrous web while fluid jets rearrange the fibers and entangle them in their new arrangement to provide a stable fabric. The support member must not undergo any substantial distortion under the force of the fluid jets. Also, the support member must have means for removing the relatively large volumes of entangling fluid so as to prevent "flooding" of the fibrous web, which would interfere with effective entangling. Typically, the support member includes drainage apertures which must be of a sufficiently small size to maintain the integrity of the fibrous web and prevent the loss of fiber through the forming surface. In addition, the support member should be substantially free of burrs, hooks or the like irregularities that could interfere with the removal therefrom of the entangled fabric. At the same time, the support member must be such that fibers of the fibrous web being processed thereon are not washed away under the influence of the fluid jets.
While machining may be used to fabricate such topographical support members, such a method of manufacture is extremely expensive and often results in aforementioned burrs, hooks and irregularities. Thus, there is a need for a method for making topographical support members which method is less expensive and reduces the numbers of burrs, hooks and irregularities therein.
Vacuum perforated plastic films are well known in the art and have been used for many years as a cover stock for sanitary napkins, disposable diapers, and assorted absorbent products for wound dressings and the like.
An exemplary method and apparatus for vacuum perforating of unperforated thermoplastic film, e.g., a polyethylene film, is disclosed in U.S. Pat. No. 4,806,303 issued to Bianco et al. on Feb. 21, 1989, the disclosure of which patent is hereby incorporated by reference. The perforation apparatus of Bianco et al. comprises a generally cylindrical metallic forming die having a plurality of holes therein. In general, the plurality of holes in the forming die corresponds to the pattern of perforating which is intended to be provided in the thermoplastic film. Such forming dies were typically made by techniques which involved photoengraving, electrodeposition of nickel and mechanical perforation. This type of forming die had a thickness on the order of a millimeter or even less, a diameter on the order of 500 millimeters and a length on the order of a meter or more. In view of the foregoing dimensions, such forming dies lacked intrinsic rigidity. Prior to the development of Bianco et al., such forming dies were supported by means of an internal stiffening drum or cylinder. Among other things, such internally located support members tended to limit the pattern of perforations which could be provided in the forming die and/or obstructed the flow of air through the vacuum forming apparatus during manufacture of the perforated film. Bianco et al. provides a rotatable cylindrical body 4 comprising a perforated band 6, which functions as the forming die, and a pair of annular end bodies 5 each of which includes a circular flanged edge 9. Each circular flanged edge 9 is toothed and meshes with a motor driven wheel 11. Axially adjustable jaw members 13, 14 at each end of the rotatable cylinder 4 engage the aforementioned pair of flanged edges and exert a pulling action on the cylindrical body to cause what is described as a "dynamic" stiffening of die 6. As a result of this dynamic stiffening, die 6 behaves as a rigid body capable of rotating about its longitudinal axis without being subjected to considerable torsional stresses. This dynamic stiffening also prevents excessive flexing of the die 6 into the vacuum slot of the vacuum forming apparatus. The jaw arrangement which provides the dynamic stiffening of the cylindrical body in Bianco et al. not only requires maintenance and adjustment, but adds appreciably to the stresses to which the forming die is subjected, dramatically reducing its life.